Hydraulic machines such as dozers, loaders, excavators, backhoes, motor graders, and other types of heavy equipment use one or more hydraulic actuators to accomplish a variety of tasks. These actuators are fluidly connected to a pump of the machine that provides pressurized fluid to chambers within the actuators, and also connected to a sump of the machine that receives low-pressure fluid discharged from the chambers of the actuators. As the fluid moves through the chambers, the pressure of the fluid acts on hydraulic surfaces of the chambers to affect movement of the actuators. A flow rate of fluid through the actuators corresponds to a velocity of the actuators, while a pressure differential across the actuators corresponds to a force of the actuators.
Control over the speed and/or force of hydraulic actuators can be provided by way of one or more metering valves. For example, a first metering valve controls fluid flow into a head-end of a hydraulic cylinder, while a second metering valve controls fluid flow out of the head-end. Likewise, a third metering valve controls fluid flow into a rod-end of the hydraulic cylinder, while a fourth metering valve controls fluid flow out of the rod-end. The different metering valves are cooperatively opened and closed (e.g., based on operator input) to cause fluid to flow into one end of the hydraulic cylinder and simultaneously out of an opposing end, thereby extending or retracting the hydraulic cylinder.
A conventional metering valve includes a body having a bore that receives a spool, and two or more passages formed in the body that communicate with each other via the spool. The spool is generally cylindrical, and includes lands that extend outward away from the body on either side of a valley or annular groove. When the lands are positioned at one or more entrances of the passages, the spool is in a flow-blocking position. When the spool is moved to a flow-passing position, the valley bridges the entrances such that fluid communication between the passages is established via the valley.
Conventional valves can be massive and require a significant amount of energy to move them quickly between the flow-blocking and flow-passing positions. For this reason, pilot valves are often connected to opposing ends of the spool, and function to selectively communicate pressurized pilot fluid with the spool ends. The pilot fluid functions to urge the spool between positions. Using multiple pilot valves, however, can be expensive and consume a large amount of space in the associated valve block.
One attempt to address the issues discussed above is disclosed in U.S. Pat. No. 7,422,033 (the '033 patent) by Barber that issued on Sep. 9, 2008. In particular, the '033 patent discloses a valve assembly having a control spool, a centering spring located at a first end, and a single pilot valve located at a second end of the control spool. The pilot valve includes a tubular sleeve, a pilot spool slidably received within the sleeve, and a dual-action stepper motor connected to move the pilot spool. The sleeve has a projection that engages a piston connected to an end of the control spool. The stepper motor is selectively actuated in one of two different directions to move the pilot spool in opposing directions. As the pilot spool slides within the sleeve, opposing sides of the piston are fluidly connected with either a tank pressure or a supply pressure, thereby causing the control spool to move. As the control spool moves, the sleeve of the pilot valve riding on the piston also moves to provide position feedback to the pilot valve.
Although the valve assembly of the '033 patent may benefit from having only a single control valve and a dual-action stepper motor, the valve assembly may still be complex, expensive, and bulky. Specifically, the use of the piston and sleeve may increase a component count, component cost, and assembly difficulty. Further, the valve assembly may lack adequate force feedback, and the in-line location of the piston and spring at opposing ends of the control spool may increase a length of the valve assembly. In addition, the stepper motor used in the '033 patent may lack the durability required for some applications.
The disclosed valve and pilot actuator are directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.